IMHO the problem with floats is they can end up not floating (which is not TOO bad, at least they end up reading "empty" all the time; worse if the mechanism jams & it reckons you have fuel when you don't... Plus, it's an electromechanical system, and on cars, anything mechanical will eventually break...
Weight's no good, because the weight transfer as the car moves around on the road, goes up/down hills and bumps, will always be throwing off the readings. Hit a big bump with a 1/2 full tank and one moment it'll read full, the next empty, then full again until all the oscillations in the suspension and the fuel itself damp down...
Nah, I have to say, I like the idea of a capacitive sensor. No moving parts, once the capacitance range is determined, and any temperature offset compensated for; it should be extremely reliable, long lasting, high resolution and very accurate. The 2nd tube idea is great, if there's room in the tank for it, but it should still be possible to use an arbitrary off-the-shelf capacitor which, once the circuit is tuned, should be just as good as a 2nd tube, and also much cheaper and smaller.